There are known a number of plasma chemical vapor deposition methods (hereinafter referred to as "plasma CVD method") and a number of apparatuses for practicing those plasma CVD methods (hereinafter referred to as "plasma CVD apparatus").
As for the plasma CVD methods, as they have advantages such as (i) capable of forming a deposited film at a low temperature of from 200.degree. to 400.degree. C., (ii) requiring no heat resistance for a substrate, etc. In view of this, they have been applied for the formation of a silicon dioxide film or a silicon nitride film which is usable as an insulating film in semiconductor systems, an amorphous silicon (a-Si) film which is usable in solar cells, close-contact type image sensors or photosensitive drums, a diamond thin film, etc. As for the plasma CVD apparatuses, there has been predominantly used a radiowave (RF) plasma CVD apparatus that RF is introduced between two flat plate type electrodes opposed in parallel with each other, thereby forming a plasma. And for such RF plasma CVD apparatus, it has a merit that its size may be easily modified because of their simple structure.
However, the conventional RF plasma CVD method involves the following drawbacks on the other hand. That is, ion sheath is apt to form on the side of a substrate to develop a negative self-bias, by which ion species in the plasma is drawn to the cathode to moderate the incident impact shock of the ion species to the anode on which the substrate is disposed. However, the ion species is still applied to the surface of the substrate and mixed into the deposited film to bring about internal stresses or increase the defect density, failing to obtain a deposited film of good quality. In addition, since the electron density is as low as from 10.sup.8 to 10cm.sup.-3 decomposing efficiency of the starting material gas is not so high and the deposition rate is low. Further in addition, since the electron temperature is as low as -4 eV, starting material gas of high bonding energy such as silicon halide compound is less decomposable.
For improving the above-mentioned drawbacks of the RF plasma CVD method, there have been proposed, in recent years, several plasma treating methods and apparatuses used therefor using a microwave of about 2.45 MHz capable of efficiently forming a high density plasma and, at the same time, heating an object to be treated. And studies have been made on the method of depositing a thin film such as silicon dioxide, silicon nitride, a-Si, diamond, etc. as well as an etching method for the silicon film.
Incidently, conventional microwave plasma treating apparatuses are classified roughly into two types.
One of them is of a type as disclosed in Japanese Patent Publication Nos. 58-49295 and 59-43991 and Japanese Utility Model Publication No. 62-36240, in which a gas pipe is inserted through or placed in contact with a rectangular or coaxial waveguide to form a plasma (hereinafter referred to as "type 1 MW-plasma treating apparatus").
The other one is a type as disclosed in Japanese Patent Laid-Open No. 57-133636, in which electron cyclotron resonance (ECR) is established within a cavity resonator and a plasma is drawn out by a divergent magnetic field (hereinafter referred to as "type 2 MW-plasma treating apparatus").
FIG. 3 shows a typical type 1-MW-plasma treating apparatus (refer to Japanese Utility Model Publication No. (62-36240).
That is, the type 1 MW-plasma treating apparatus comprises a vacuum system, an exhaust system and a microwave introducing system as shown in FIG. 3.
Referring to FIG. 3, the exhaust system comprises an exhaust pipe 307B and an exhaust pump 308. The vacuum system comprises a reactor 307, the exhaust system and vacuum gauge 313. The reactor 307 connects to a first gas introducing pipe 307a including a microwave transmissive tube (for example, made of quartz tube) having an inside diameter on the order of 40 mm.
The microwave introducing system comprises a microwave waveguide connected to a microwave power source 301, and isolator 302, microwave power monitor 303 for detecting reflected electric energy provided with an indicator 303a, impedance matching unit 304, shielding pipe 305a, a sliding short-circuit plate 305, i.e. a plunger 306.
The microwave transmissive quartz tube is connected to the gas introducing pipe 307a and is arranged perpendicularly to the microwave waveguide.
A second gas introducing pipe (not shown) is connected to the inside of the reactor 307 and a gas (silane gas) supplied is exhausted through an exhaust system (307b and 308). In the apparatus, the gas introduced through the first gas introducing pipe (O.sub.2 gas or N.sub.2 gas) is converted into plasma by microwave discharge. During microwave discharge caused by microwave energy, microwave input impedance can be matched by moving a plunger 306.
Radicals of the plasma thus produced react with the silane gas supplied through the second gas introducing pipe whereby a silicon dioxide film or a silicon nitride film is formed on the surface of the substrate 309.
FIG. 4 shows a typical type 2 MW-plasma treating apparatus (refer to Japanese Patent Laid-Open No. 57-133636). The system and configuration of this apparatus are the same as those of the foregoing type 1 MW-plasma treating apparatus except for the discharging space for which an electromagnet 407 is used. That is, the vacuum system comprises a cylindrical plasma producing vessel 401 and a reactor 402 connected thereto, in which a microwave introducing window 403 is attached hermetically to the plasma producing vessel. A first gas introducing pipe 406 and a microwave waveguide 404 are connected to the plasma producing vessel 401. The plasma producing vessel 401 is watercooled by means of a water-cooling pipe 405 disposed at the outer circumference thereof. The apparatus shown in FIG. 4 is provided with an electromagnet 407 disposed coaxially with the plasma producing vessel 401. The direction of lines of magnetic force from the electromagnet 407 is the same as the travelling direction of the microwave. Electrons move for a magnetron motion by the combination of a magnetic field and an electric field formed by the microwave in the perpendicular direction. Therefore, the plasma producing vessel 401 is designed as a cavity resonator of a TE.sub.11t mode (t=a natural number). A second gas introducing pipe and the exhaust system are connected to the reactor 402, and gases within the deposition vessel are exhausted by the exhaust system.
When the typical type 2 MW-plasma treating apparatus shown in FIG. 4 is used, for example, as a deposition apparatus a gas (H.sub.2 gas) introduced through the first gas introducing pipe 406 is formed into a plasma by electric discharge caused by the microwave energy. When the magnetic flux density of the magnetic field is 875 gauss, the reflected wave of the microwave energy is almost zero. In this apparatus, the end plate 411 of the cavity resonator having the construction of a choke is moved under vacuum depending on the type of the gas, the pressure of the gas and the microwave power applied, so that the cavity resonator meets required conditions. Radicals in the plasma are transported under an electron cyclotron motion in the direction of the lines of magnetic force, and the radicals in the plasma react with the gas (silane gas) introduced through the second gas introducing pipe to form an a-Si film over the surface of a substrate 408.
However, both the type 1 MW-plasma treating apparatus and the type 2 MW-plasma treating apparatus have the following problems to be solved.
That is, the type 1 MW-plasma treating apparatus involves the following drawbacks; (i) it is necessary to control the pressure during electric discharge to an order of about 0.05 Torr or higher, or to a gas of such a type as having a large ionizing cross sectional area easily causing electric discharge in order to attain stable discharge: and (ii) in the case where the apparatus is used for the film deposition, when the charged microwave power is increased in order to increase the film deposition rate, an electric field is concentrated to the junction between the quartz tube and the waveguide to cause sputtering to the quartz tube, by which impurities formed by the sputtering are mixed into the deposited film, failing to obtain a deposited film of satisfactory property.
On the other hand, the type 1 MW-plasma treating apparatus is free from the sputtering problem as described above and discharge even in a low pressure region of about 10.sup.-4 Torr is possible. However, there are the following problems upon forming, for example, an a-Si film by using a H.sub.2 gas and a silane gas (SiH.sub.4); (iii) an a-Si film is deposited on the microwave introducing window along with the progress of the depositing reaction making it difficult for impedance matching and maintenance of electric discharge: (iv) since the microwave introducing window 403 and the waveguide 404 are fastened and fixed, the end plate 411 has to be moved in vacuum for changing the axial length of the cavity resonator, thereby making the operation difficult: and (v) the weight of the apparatus is heavy and the cost is expensive since the apparatus uses coils for generating electric field under ECR conditions.